The Science Underpinning Anomalous Scaling Laws of Strength and Toughness in Nanocellulose Materials

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The quest of materials possessing both high strength and high toughness is perpetual in advanced material design. Unfortunately, these two mechanical properties are generally mutually exclusive. For example, for metals and alloys, their toughness is usually inversely proportional to their strength. As a result, in practice, the design of strong and tough materials is inevitably a compromise. So far, a general and feasible mechanism to address the conflict of strength vs. toughness still remains elusive. This project plans to shed insight on a possible strategy resolving the conflict of strength vs. toughness by investigating the fundamental science of the anomalous but desirable scaling law of mechanical properties of cellulose nanopaper: both its strength and toughness increase as the cellulose fiber size decreases. These unique features combined with the intrinsic biodegradability, low-cost and scalable manufacturing of cellulose paper can have significant impact on bottom-up design of high performance materials with desirable mechanical properties. The research plan will be carried out in a systematic framework integrating multi-scale mechanics modeling and complementary experiments and consists of two thrusts. The overarching goal of Thrust 1 is to establish a quantitative understanding of the underpinning mechanism of both strong and tough cellulose nanopaper, by investigating the mechanics of cellulose molecular chains, nanofibrillated cellulose fibers and cellulose fiber network via multi-scale mechanics modeling and complementary experiments. The fundamental understanding from Thrust 1 will lay out a solid foundation that motivates and enables the research efforts in Thrust 2, which focuses on generalizing the underpinning mechanism of attaining both strength and toughness to guide the exploration of a class of strong and tough materials with other highly desirable multi-functionalities. Interdisciplinary graduate student training will enable to access world-class imaging and metrology facilities at NIST, and to learn from the vast expertise and state-of the-art technology from a world leading paper company. Outreach activities will be conducted via iMechanica.org, the largest international online community of mechanics with 50,000+ registered users.